In recent years voltage-controlled oscillators (VCOs), which can control oscillating frequencies by applying a control voltage, have widely been used for, for example, local oscillators of wireless transmitter-receivers. Such a voltage-controlled oscillator is required to be highly stable, have low phase noise, generate low noise, or the like. However, since the range of oscillating frequencies of a wireless transmitter-receiver is wide, the aforesaid requirements cannot be satisfied by a single voltage-controlled oscillator. Taking into account of this, a plurality of voltage-controlled oscillators with different variation ranges of oscillating frequencies are provided on one integrated circuit, in order to cover a required oscillating frequency range.
In case where a plurality of voltage-controlled oscillators are provided on an integrated circuit, the chip size increases on account of an area for those oscillators, so that the cost is high. Taking account of this problem, there is a voltage-controlled oscillator which can cover a required oscillating frequency range as if different voltage-controlled oscillators are provided. In this voltage-controlled oscillator, variation ranges of oscillating frequencies are switched by switching the inductance of a resonance circuit.
FIG. 5 is a circuit diagram which outlines a voltage-controlled oscillator 500 with the inductance switching capability. This voltage-controlled oscillator 500 is disclosed in Japanese Laid-Open Patent Application No. 2003-229718 (published on Aug. 15, 2003; corresponding to U.S. Pat. No. 6,954,111). As illustrated in FIG. 5, the voltage-controlled oscillator 500 includes a pair of variable-capacity elements C501 whose capacity control terminals are connected to a control voltage input terminal 501. To this control voltage input terminal 501, a control voltage is applied from the outside. With this, the capacities of the variable-capacity elements C501 are changed, so that the resonance frequency of a resonance circuit including inductors L501 and variable-capacity elements C50-1 is controlled. Moreover, the voltage-controlled oscillator 500 includes switches SW501 by which a power source 502 is connected to either terminals of the inductors L501 or intermediate parts of the inductors L501. In this voltage-controlled oscillator 500, the inductance of the resonance circuit is varied by switching the switches SW501, with the result that the variation range of oscillating frequencies is switched.
FIG. 6 is a circuit diagram which outlines another voltage-controlled oscillator 600 having the inductance switching capability. This oscillator 600 is disclosed in Japanese Laid-Open Patent Application No. 2002-151953 (published on May 24, 2002). As illustrated in FIG. 6, the voltage-controlled oscillator 600 includes a pair of variable-capacity elements C601 whose capacity control terminals are connected to a control voltage input terminal 601. A control voltage is applied to the control voltage input terminal 601 from the outside, so that the capacities of the variable-capacity elements C601 are varied. In doing so, the resonance frequency of the resonance circuit including inductors L601 and the variable-capacity elements C601 is controlled. Furthermore, as shown in FIG. 6, each main inductor L601 is provided with an auxiliary inductor L602 which is magnetically coupled with the main inductor L601. On this account, in the voltage-controlled oscillator 600, the inductance of the resonance circuit is changed by switching on/off the switches SW601 which are connected to the closed circuit including the auxiliary inductors L602, so that the variation range of the oscillating frequencies is switched.
However, the conventional voltage-controlled oscillator in which the variation range of the oscillating frequencies is switched by switching the inductance of the resonance circuit has the following problem.
That is, in the voltage-controlled oscillator 500, the switches SW501 and the inductors L501 are connected in series. On this account, the currents passing through the switches SW501 and the inductors L501 are susceptible to a parasitic capacity and noise, and hence the phase noise characteristic is deteriorated and noise is increased.
In the meanwhile, in the voltage-controlled oscillator 600, the auxiliary inductors L602 by which mutual magnetic induction with the main inductors L601 is generated are required, in addition to the main inductors L601. For this reason, the voltage-controlled oscillator 600 requires an area where the inductors L601 and L602 are provided, and hence the chip size is large. To solve this problem, there has been a proposal to provide two or more wiring layers where those inductors are formed. In this case, however, it is necessary to provide, between the neighboring wiring layers, a ground layer (termed ground shield) made of polysilicon or the like. This increases manufacturing costs.
In consideration of the above, there has been a voltage-controlled oscillator in which the variation range of the oscillating frequencies is switched by switching the capacity of the resonance circuit. FIG. 7 is a circuit diagram which outlines a voltage-controlled oscillator 700 having a capability of switching the capacity of a resonance circuit. As shown in this figure, the voltage-controlled oscillator 700 includes a pair of variable-capacity elements C701 whose capacity control terminals are connected to a control voltage input terminal 701. As the control voltage input terminal 701 receives a control voltage, the capacities of the variable-capacity elements C701 are changed, so that the resonance frequency of the resonance circuit including inductors L701 and the variable-capacity elements C701 is controlled. Furthermore, the voltage-controlled oscillator 700 is also provided with another pair of variable-capacity elements C702. The capacity control terminal of each of the variable-capacity elements C702 is connected to either the GND or the power source, via the switch SW701. With this arrangement, the capacities of the variable-capacity elements C702 are switched by a switch SW701, so that the variation range of the oscillating frequencies is switched.
The voltage-controlled oscillator 700 does not require an inductor for switching the variation range of the oscillating frequencies. For this reason, the voltage-controlled oscillator has a good phase noise characteristic can be easily downsized.
The voltage-controlled oscillator, in which the variation range of the oscillating frequencies is switched by switching the capacity of the resonance circuit, has a good phase noise characteristic and can be easily downsized, but has a problem such that the ratio between the oscillating frequencies before and after the switching of the variation range of the resonance frequency is not constant, because of the dependency on the control voltage. When such a voltage-controlled oscillator is used as a local oscillator of a transmitter or a receiver, the transmitter or the receiver requires complicated circuitry, and hence is costly.
The above-mentioned problem is specifically described as below, taking the voltage-controlled oscillator 700 shown in FIG. 7 as an example.
The resonance circuit of the voltage-controlled oscillator 700 is constituted by inductors L701, variable-capacity elements C701 which are connected in parallel with the inductors L701, and other variable-capacity elements C702. The total capacity C of the resonance circuit is therefore equal to CV+CX, which is the sum total of (i) the capacity CV of the variable-capacity elements C701, which is determined by a control voltage Vc applied to a control voltage input terminal 701 and (ii) the capacity CX of the variable-capacity elements C702.
As described above, the capacity CX of the variable-capacity elements C702 is switchable using the switch SW701. That is, in case where the capacity control terminal of the variable-capacity elements C702 is grounded, the capacity of the variable-capacity elements C702 is CA. On the other hand, in case where the capacity control terminal of the variable-capacity elements C702 is connected to the power source, the capacity of the variable-capacity elements C702 is CB(CA>CB).
The oscillating frequency of the voltage-controller oscillator, i.e. the resonance frequency f of the resonance circuit is determined as below, by the total capacity C of the resonance circuit and the inductance L of the resonance circuit:
  f  =      1          2      ⁢                          ⁢      π      ⁢                          ⁢                        L          ·          C                    
Therefore, when the capacity of the variable-capacity elements C702 is CA, the resonance frequency fA of the voltage-controlled oscillator 700 is determined as below:
      f    A    =      1          2      ⁢                          ⁢      π      ⁢                          ⁢                                    L            ·                          (                                                C                  V                                +                                  C                  A                                            )                                ⁢                                                    
In the meanwhile, when the capacity of the variable-capacity element is CB, the resonance frequency fB of the voltage-controlled oscillator 700 is determined as below:
      f    B    =      1          2      ⁢                          ⁢      π      ⁢                          ⁢                        L          ·                      (                                          C                V                            +                              C                B                                      )                              
In the equations above, indicated by L is the inductance of the inductors L701.
When the variation range of the resonance frequency is switched while the control voltage Vc is kept constant, the ratio between the resonance frequencies fA/fB before and after the switching is represented as below.
            f      A              f      B        =                              C          V                +                  C          B                                      C          V                +                  C          A                    
In this manner, the ratio between the resonance frequencies before and after the switching of the resonance frequency range depends on the capacity CV, i.e. the control voltage Vc. The ratio is therefore inconstant.
The present invention was done to solve the above-described problem. The objective of the present invention is to provide a voltage-controlled oscillator (i) which has a good phase noise characteristic, (ii) which can be easily downsized, and (iii) whose ratio of oscillating frequencies before and after the switching of the variation range of a resonance frequency does not depend on a control voltage.